1. Field of the Invention
The present invention relates generally to heat exchanging apparatus and, more particularly, to water heaters.
2. Description of the Prior Art
The most common type of water heater presently in use consists of a storage tank for retaining heated water combined with some means for heating the water. Perhaps the most common means for heating the stored water is that in which a hot flame, produced by gas, for example, causes hot gases, including the combustion products, to rise through an elongated flue passing through the central portion of the storage tank. In this particular type of water heater the hot gases flowing up the flue contact its inner surface while the water within the storage tank is in contact with the flue's outer surface. While combustion continues, the water within the storage tank is heated by thermal conduction through the wall of the flue. In this type of water heater, an elongated baffle generally is disposed within the flue, about its center and along its length, to increase the efficiency of heat transfer from the hot gases through the wall of the flue to the water within the storage tank. Generally, this type of water heater includes a thermally activated valve that halts a combustion or main burner when the temperature of the water within the storage tank reaches a pre-established temperature.
To reduce heat loss from the water heater both while the water is being heated and after combustion has been cut off, the exterior surface of the storage tank is typically covered with a thermal insulating material. However, the inner surface of the flue cannot be thermally insulated because it would inhibit heat transfer while the water is being heated. Unfortunately, once the water becomes heated and the main burner combustion stops, heat from the water is transferred back through the wall of the flue and heats the upwardly flowing air and gas within the flue associated with a pilot burner, for example, or induced air flow from the room area surrounding the heater. This heating of the air within the flue is a significant source of heat loss in water heaters and, consequently, reduces their efficiency and increases their operating cost.
In most water heater installations, a vent pipe connects to the upper outlet of the flue outside of the water heater to conduct combustion products out of the building in which the heater is located. One technique for reducing heat loss from water heaters due to heating of the air within the flue during intervals when there is no combustion is the installation of a temperature responsive damper in this vent pipe. Examples of different types of such temperature responsive vent dampers are set forth in U.S. Pat. Nos. 3,197,139, 4,294,401, 4,337,892 and 4,384,671. However, because the flue's vent pipe must remain at least partially open for venting combustion products from the water heater's pilot which burns continuously whether or not water is being heated, the various devices disclosed in the preceding patents cannot completely block the flue's vent pipe to cut off all air and gas flowing through the flue.